Peter Mandelson has accused the prime minister of "unreasonable and irresponsible" behaviour in the way he granted concessions to Sinn Féin during Downing Street's attempts to broker a peace deal in Northern Ireland. As Mr Blair tries once again to revive power sharing, he is criticised by one of his closest political allies of "conceding and capitulating" to republican demands, which alienated unionists.

In a Guardian interview for a series examining the prime minister's handling of the peace process, the former Northern Ireland secretary praised Mr Blair for his commitment to the process, dating back to when he became Labour leader in 1994. But he added: "In order to keep the process in motion [Tony] would be sort of dangling carrots and possibilities in front of the republicans which I thought could never be delivered, that it was unreasonable and irresponsible to intimate that you could when you knew that you couldn't."

Mr Mandelson's revelation that he disagreed with the prime minister - at one point he refused an order to write a secret letter to Sinn Féin - sheds new light on his second resignation from the cabinet in 2001. At the time of his departure Alastair Campbell, then the prime minister's official spokesman, openly questioned Mr Mandelson's judgment over Northern Ireland on the grounds that he became overly sympathetic to the unionists and too hostile to Sinn Féin.

Downing Street officials interviewed by the Guardian say that Mr Blair has wrestled with the dilemma highlighted by Mr Mandelson over the past decade: how to bring Sinn Féin in from the cold without destroying unionist support. Lord Butler , the former cabinet secretary, says: "There was a lot to be said for paying a price to keep the bicycle moving. The issue is whether Tony Blair paid too big a price."

Lord Butler and Mr Mandelson are among a series of senior officials and politicians - including all four surviving Northern Ireland secretaries to have served under Mr Blair - whose interviews appear in this week's three-day Guardian series on the peace process. Political leaders from across the spectrum, including the former Ulster Unionist leader Lord Trimble and Sinn Féin's chief negotiator Martin McGuinness, praise Mr Blair for his commitment to Northern Ireland.

Mr McGuinness hails the prime minister for ending the "Thatcher mentality" on the issue. His favourable views are not shared by Seamus Mallon, the SDLP's former deputy first minister, who tells the Guardian that Mr Blair treated the late Mo Mowlam "like shit", employing an approach in which the prime minister would "buy anybody [and] sell anybody".

The Guardian series sheds new light on the peace process by revealing:

· Downing Street believed that the IRA leadership ordered the United Kingdom's biggest bank robbery in 2004 from the Northern Bank after the political process hit the rocks;

· Peter Mandelson says ministers had to maintain a "fiction" that they were not talking to the IRA when they met Sinn Féin;

· John Reid, the home secretary, believes the IRA were targeting individuals for attack as recently as 2002;

· George Mitchell, the former US senator who chaired the Good Friday agreement talks in 1998, warns of continuing crises even if a power-sharing deal is reached by the end of this month;

· The prime minister used his Protestant Ulster roots - his maternal grandfather was a member of the Orange Order - to woo unionists but said nothing of his background to nationalists.

The revelations come as the prime minister tries to broker a power-sharing deal between Sinn Féin and the DUP after the two parties dominated last week's election to the Northern Ireland assembly.

Mr Mandelson reveals that Sinn Féin lay at the heart of his row with the prime minister just a month after he succeeded Mo Mowlam as Northern Ireland secretary in October 1999. The prime minister demanded that Mr Mandelson write a secret letter to Sinn Féin offering a form of amnesty to IRA fugitives, known as "on-the-runs", among other "sweeties".

"I was at a performance of the Royal Ballet visiting Belfast and I was taken out three times during the performance to talk to No 10 about this," Mr Mandelson said. "I said ... I am not prepared to do it because I have my own standing to think of and a secret side letter is not how I want to do business. They came back and said that the prime minister takes a different view, that you do need to make these offers to the republicans and he wants you to write this letter. I said if the prime minister wants to make these offers I am afraid he will have to write his own letter."

The letter was sent and the concessions were formally offered to Sinn Féin at the Weston Park talks in July 2001 six months after Mr Mandelson left office. "Weston Park was basically about conceding and capitulating in a whole number of different ways to republican demands - their shopping list. It was a disaster because it was too much for them ... That was a casualty of my departure, I would say." Mr Mandelson added: "When Gerry Adams and Martin McGuinness entered the room you were expected to stand up. They were senior military, they were top brass. Apart from being leaders of Sinn Féin they were leaders of the military council."

The Sinn Féin MPs have always denied being IRA leaders.

John Reid, Mr Mandelson's successor in Northern Ireland, is more supportive of Downing Street's efforts, saying: "If Tony Blair's Labour government never did anything else but bring to an end the longest-running political dispute in European history and the longest running war probably in world history, on and off, it would be worth having the Labour government just for that."